Direction finding antenna system

ABSTRACT

A direction finding antenna system having a large circle of active reflector antenna elements and a smaller concentric circle of focal point active antenna elements. The outer circle consists of several hundred antenna elements where each element comprises a horizontal one-half wave length horizontal dipole and a onefourth wave length vertical dipole. The horizontal dipole receives the incoming signal the output of which is amplified and applied to the input of the vertical dipole which then retransmits the signal. A group of reflector antenna elements forming a segment of the outer circle receive and retransmit the incoming signal to a focal point where the signal is received by a focal point antenna on the inner circle. The signal from the focal point antenna receiving the retransmitted signal thus represents the output of a directional antenna. A display of signal arrival direction is obtained from the output of the focal point antennas.

United States atent Barry 51 Nov. 28, 1972 DIRECTION FINDING ANTENNASYSTEM Inventor: George H. Barry, Saratoga, Calif.

Assignee: The United States of America as represented by the Secretaryof the Navy Filed: June 2, 1970 Appl. No.: 42,640

References Cited UNITED STATES PATENTS 10/1968 Tillotson ..343/ 100 ST2/1971 l-limmel et al....343/10O SA X 7/1962 Russell ..343/100 SA UX4/1966 McFarland ..343/1OO SA X Primary Examiner-Benjamin A. BorcheltAssistant Exandner-Richard E. Berger Att0meyR. S. Sciascia and CharlesD. B. Curry A direction finding antenna system having a large circle ofactive reflector antenna elements and a smaller concentric circle offocal point active antenna elements. The outer circle consists ofseveral hundred antenna elements where each element comprises ahorizontal one-half wave length horizontal dipole and a one-fourth wavelength vertical dipole. The horizontal dipole receives the incomingsignal the output of which is amplified and applied to the input of thevertical dipole which then retransmits the signal. A group of reflectorantenna elements forming a segment of the outer circle receive andretransmit the incoming signal to a focal point where the signal isreceived by a focal point antenna on the inner circle. The signal fromthe focal point antenna receiving the retransmitted signal thusrepresents the output of a directional antenna. A display of signalarrival direction is obtained from the output of the focal pointantennas.

7 Claims, 5 Drawing Figures PATENTEMuvza I972 SHEET 1 [1F 2 F, INVENTORGEORGE H. BARR) f 6W7 ATTORNEY PAIENTED NDY28 I972 3. 704.463

sum 2 or 2 GROUND PLANE GROUND PLANE Fig. 4A F/ ...-4B Y DIRECTIONFINDING ANTENNA SYSTEM The invention described herein may bemanufactured and used by or for the Government of the United States ofAmerica for governmental purposes without the direction finding of anyroyalties thereon or therefor.

The present invention relates to an antenna system and more particularlyto a large size direction finding reflector antenna system.

Currently used large size communications and direction-finding antennastypically have apertures on the order of 300 meters and beamwidths offrom 2 to in the HF frequency range of from 30 down to 6 MHz. When HFsignals are received on scanning arrays of this size it has been foundthat the pattern of the received signal energy traces out the responsepattern of the antenna itself. Therefore, larger apertures could bequite useful to provide higher system gain and more precise signaldiscrimination.

One of the diff culties encountered with larger arrays is that theircost increases approximately as the square of their resolution. As thesize of array increases, the cost of the elements themselves becomesinsignificant as compared to, the cost of the cables required toassemble all of the signal samples and the cost of processing thesignals to form beams. For either circular or linear arrays, the cablecost increases with the square of the aperture or the number ofelements. The cost of signal processing also increases with the squareof the aperture. As the number of elements increases more signals mustbe processed to form each beam, and more beams are required to cover agiven sector.

The present invention overcomes the square law cost problem associatedwith existing large antenna systems by providinga very effective antennasystem that has a cost factor that only increases linearly with thesize.

Briefly, the present invention comprises a large circle of activereflector antenna elements and a smaller concentric circle of focalpoint antenna elements. A group of reflector antenna elements forming asegment of the outer circle receives and retransmits the incoming signalto a focal point where the signal is received on one of the focal pointantennas on the inner circle. The signal from this focal point antennaestablishes the direction from which the incoming signal is coming. Theouter circle may consist of several hundred antenna elements, forexample, where each element comprises a one-half wave length horizontaldipole and a vertical dipole having a wave length of one-fourth wavelength. The horizontal dipole receives the incoming signal. The outputof the horizontal dipole is amplified and applied to the input of thevertical dipole which then retransmits the signal. Vertical quarter wavelength dipoles are used as focal point antennas to receive theretransmitted signals. Each of the focal point antennas are connected tosignal processing equipment for rapid analysis.

Other objects, advantages and novel features of the invention willbecome apparent from the following detailed description of the inventionwhen considered in conjunction with the accompanying drawings wherein:

FIG. 1 is a plan view of the antenna system of the present invention;

FIG. 2 is an enlarged segment of the antenna system of FIG. I;

FIG. 3 is a schematic drawing of one of the active reflection antennaelements of FIGS. 1 and 2;

FIG. 4A is a diagram illustrating the radiation pattern of a one-fourthwave length dipole; and

FIG. 4B is a diagram illustrating the radiation pattern of a one-halfwave length dipole.

In FIG. I is illustrated a plan view of the antenna system of thepresent invention. This antenna system includes a roundhouse 11 whichincludes equipment for processing the received signals, a circular arrayof active reflector antenna elements 13, a circular array of low beamfocal point antennas 15, and a circular array of high beam focal pointantennas 17.

The incoming signal rays are shown by the parallel lines indicated byreference numeral 19. These parallel rays 19 have a wave frontillustrated by parallel lines 21 which are perpendicular to parallelrays 19. The incoming wave front 21 which are perpendicular to parallelrays 19. The incoming wave front 21 of rays 19 are received and thenreflected by reflector antenna elements 13, within segment B, to formreflected rays 23 having wave fronts 25 which are focused at point A ofthe low beam focal point antennas 15. The signal that is received by thefocal point antennas are transmitted by coaxial cable 26, for example,back to the roundhouse 1 1, for signal processing.

In FIG. 2 is illustrated a segment of the circular array between brokenlines 27 and 29 of FIG. 1 and in FIG. 3 is illustrated one of the activereflector antenna elements 13 of FIGS. 1 and 2. Referring to FIGS. 2 and3, each active reflector antenna element 13 includes a one-half wavelength dipole 31, a one-fourth wave length dipole 33, lead lines 35 and37, amplifier 39 and amplifier output line 41. Dipole 31 is one-halfwave length and feeds a balanced transmission line 35 and 37 that feedsa balanced input of amplifier 39. Amplifier 39 feeds an unbalancedone-fourth wave length dipole 33 through coaxial line 41, for example.The radiation pattern of one-half wave length dipole 31 is asillustrated by dotted lines 42. The radiation pattern of onefourth wavelength dipole 33 is illustrated by dotted lines 43. It should be notedthat the bottom of antenna 33 is mounted on the ground that forms aground plane. Dipole 31 may be mounted on a telephone pole, for example,or it may be mounted on suspended wires having electrical insulationbetween adjacent antennas. The surface of the ground plane is preferablysalt water or wire mesh.

As shown and described, antenna 31 is a one-half wave length dipole;however, it is to be understood that if broad band reception is desiredthen another type antenna, such as a log periodic antenna, may be usedin place of dipole 31. The received signal by dipole 31 is transmittedthrough lead lines 35 and 37 which is preferably a balanced transmissionline, to the input of amplifier 39. The output of amplifier 39 istransmitted through line 41, which may be of the coaxial type, to thelower end of antenna 33. The center conductor of line 41 is connected tothe base of antenna 33 and the outer conductor is connected to ground.It should be particularly noted that antenna 33, which is aretransmitting antenna, is a one-fourth length antenna. This is becausea one-fourth wave length antenna provides a radiation pattern that has astrong radiation component along the ground, as shown in FIG. 4A, sothat it will transmit a strong signal to the focal point antennas l and17. If antenna 33 were a one-half wave length antenna then there wouldbe a very weak radiation com ponent along the ground, as shown in FIG.48, so that only a very weak signal would be received by the focal pointantennas. It should be noted that antenna 33 is preferably a one-fourthwave length antenna if the received signals are within a narrow band.However, if the received signals are broad band then very little energywould be transmitted to antennas or 17 when the received signal wastwice the frequency at which antenna 33 is one-fourth wave length.Therefore, when broad band signals are received it is necessary thatantenna 33 be less than one-half wave length.

In order to prevent the component array from having large 'side lobesand" therefore" poor component directivity (one dipole 31 by itself isnot very directive) it is desirable to closely space (as indicated bydistance d of FIG. 2) the adjacent antennas 31 so that the distancebetween centers of adjacent antennas is about one wave length.

The following table sets forth the system parameters of one example ofan antenna system of the present invention. It will be obvious to oneskilled in the art that these parameters may be changed so long assystem compatibility is maintained.

TABLE I Diameter (D) 2000 meters Reflector antenna 31 spacing (d) 15meters Number of elements 13 420 Number of active elements 13* 60Reflector amplifier 39 gain l0 db at 3 MHz,

30 db at 30 MHz Receiving antenna 31 length 7% meters at MHz Receivingantenna 31 height (h) 15 meters Spacing of antennas 31 and 33 (l) 50meters Reflector antenna 33 length =4 meters at 20 MHz Focal pointantennas l5 and I7 ==4 meters at 20 MHz lengths Active elements arethose elements 13 that are within the. sector that is collecting andfocusing the received signal 'n focal point A. For example, thoseelements within the useful aperture 3" of FIG. 1. Because of sphericalabberation the elements 13 outside of aperture B will not focus at pointA.

"The size of these antennas is determined by the frequency of thereceived signals. At 20 MHz the antenna lengths are preferably asindicated. However, at a lower frequency; for example, l0 MHz, then theantenna lengths would double and the spacing d" would double.

It should be noted that the focal point A will shift radially dependingupon the elevation from which the incoming signal is received. Forexample, if the incoming signal is arriving from 0 elevation or nearlyhorizontal to the surface of the earth, as is the case for most longdistance communications, then the focal point will be on the outercircle of antennas 15. However, if the incoming signal is arriving froma elevation, for example, then the focal point will be on the innercircle of antennas 17. Additional circles of antennas similar to 15 and17 may be necessary if the received signals arrive at very high angles.However, even at very high angles, antennas 17 will still be able todetermine quite accurately the direction from which the incoming signalis arriving. This is because the center antenna will receive thestrongest signal, indicating the direction from which the signal iscoming, and the signals on either sidewill have lesser signals and bysimple plotting of the received strengths it is possible to accuratelydetermine the antenna receiving maximum signal.

What is claimed is:

1. An antenna system comprising:

a. an outer circle of a plurality of first antenna elements;

b. each of saidv first antenna elements in said outer circle of firstantenna elements includes an amplifier for amplifying an incoming signalto said antenna system;

c. an inner circle of a plurality of second antenna elements;

d. said outer circle of antenna elements receiving an incoming signalarriving from a predetermined direction and retransmitting the receivedsignal to a focal point on said inner circle of said second antennaelements; and

e. whereby said focal point identifies said predetermined direction.

2. The system of claim 1 wherein each of said first antenna elementscomprises:

a. a first dipole mounted parallel to ground for receiving said incomingsignals;

b. a second dipole mounted vertical to ground;

c. means for transmitting the incoming signal received by said firstdipoleto said second dipole; and

d. said second dipole retransmits the signal to said focal point.

3. The system of claim 2 wherein:

a. saidfirst dipole has a length of about one-half of the wave length ofsaid incoming signal.

4. The system of claim 3 wherein:

a. said second dipole has a length of less than about one-half of thewave length of said incoming signal.

5. The system of claim 4 wherein:

a. the distance between center positions of said first dipoles formingsaid outer circle is about the wave length of said incoming signal.

6. The systernof claim 5 wherein:

a. I one end of said second dipole is connected to ground.

7. The system of claim 1 wherein each of said first antenna elementscomprises:

a. a receiving element for receiving said incoming signals and having aninput and an output;

b. said amplifier including an amplifier element having an input and anoutput;

0. a retransmitting element having an input and an output; and t 'd. theinput of said amplifying element being operatively connected to theoutput of said receiving element and the input of said retransmittingelement being connected to the output of said amplifier element.

1. An antenna system comprising: a. an outer circle of a plurality offirst antenna elements; b. each of said first antenna elements in saidouter circle of first antenna elements includes an amplifier foramplifying an incoming signal to said antenna system; c. an inner circleof a plurality of second antenna elements; d. said outer circle ofantenna elements receiving an incoming signal arriving from apredetermined direction and retransmitting the received signal to afocal point on said inner circle of said second antenna elements; and e.whereby said focal point identifies said predetermined direction.
 2. Thesystem of claim 1 wherein each of said first antenna elements comprises:a. a first dipole mounted parallel to ground for receiving said incomingsignals; b. a second dipole mounted vertical to ground; c. means fortransmitting the incoming signal received by said first dipole to saidsecond dipole; and d. said second dipole retransmits the signal to saidfocal point.
 3. The system of claim 2 wherein: a. said first dipole hasa length of about one-half of the wave length of said incoming signal.4. The system of claim 3 wherein: a. said second dipole has a length ofless than about one-half of the wave length of said incoming signal. 5.The system of claim 4 wherein: a. the distance between center positionsof said first dipoles forming said outer circle is about the wave lengthof said incoming signal.
 6. The system of claim 5 wherein: a. one end ofsaid second dipole is connected to ground.
 7. The system of claim 1wherein each of said first antenna elements comprises: a. a receivingelement for receiving said incoming signals and having an input and anoutput; b. said amplifier including an amplifier element having an inputand an output; c. a retransmitting element having an input and anoutput; and d. the input of said amplifying element being operativelyconnected to the output of said receiving element and the input of saidretransmitting element being connected to the output of said amplifierelement.